1. Field of the Invention
This invention relates to methods for coating drug delivery devices. More specifically, the invention is directed to applying a composition onto an implantable medical device and drying the composition at an elevated temperature in an environment having increased relative humidity.
2. Description of the Background
In the field of medical technology, there is frequently a necessity to administer drugs locally. To provide an efficacious concentration to the treatment site, systemic administration of medication often produces adverse or toxic side effect for the patient. Local delivery is a preferred method in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Thus, local delivery produces fewer side effects and achieves more effective results.
One commonly applied technique for local delivery of the drug is through the use of medicated stents. One method of medicating a stent is with the use of a polymer coating incorporating a drug. To fabricate the polymer coating, a suitable polymer is usually dissolved in a solvent or blend of solvents, followed by applying the solution onto the stent, for example, by spraying or dipping. To complete the process of fabricating the stent coating, the stent is dried and/or baked to remove the solvent.
Examples of solvents currently used to dissolve biocompatible polymers for fabricating stent coatings include dimethylacetamide (DMAC), dimethylsulfoxide (DMSO), dimethylformamide (DMF), and formamide. These solvents or similar solvents with relatively high boiling points, for example, above 120° C. at ambient pressure, and/or low volatility, for example, having vapor pressure under 15 Torr at room temperature, have a tendency to evaporate very slowly. Prolonged period of time may be needed to allow the solvent to fully evaporate from the coating because residual or trace amounts of the solvent may remain in the coating composition, which may produce an adverse response subsequent to the stent implantation. Baking of the stent at relatively high temperatures may be needed to facilitate the process of the solvent removal. The baking temperatures used for this purpose, should not exceed the temperature at which the drug can be adversely affected, however. The embodiments of the present invention provide methods for facilitating the evaporation of the solvent from the coating composition.